The present invention relates generally to the field of ink-jet printing and, more particularly, to the delivery of ink to ink-jet print heads.
Ink-jet technology is relatively well developed. The basics of this technology are described by W. J. Lloyd and H. T. Taub in "Ink-Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No 5. (October 1988), Vol. 43, No. 4, (August 1992), Vol. 43, No.6 (December 1992) and Vol. 45. No. 1 (February 1994).
The typical thermal ink-jet print head has an array of precisely formed nozzles attached to a print head substrate that incorporates an array of firing chambers that receive liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor", located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal ink-jet firing resistor, a small portion of the ink near it vaporizes and ejects a drop of ink from the print head. The nozzles are arranged in a matrix array. Properly sequencing the operation of each nozzle causes characters or images to form on the paper as the print head moves past the paper.
In an effort to reduce the cost and size of ink-jet printers and to reduce the cost per printed page, engineers have developed ink-jet printers having small, moving print heads that are connected to large stationary ink reservoirs by flexible ink tubes. This development is called "off-axis" printing. In such printers the mass of the print head is sharply reduced so that the cost of the print head drive system and the over all size of the printer can be minimized. In addition, by separating the ink reservoir from the print head has allowed the ink to be replaced as it is consumed without requiring frequent replacement of costly print heads.
With the development of off-axis printing has come the need for numerous flow restrictions to the ink between the ink reservoir and the print head. These restrictions include additional orifices, narrow conduits, and shut off valves. To over come these flow restrictions and also to provide ink over a range of printing speeds, ink is now transported to the print head at an elevated pressure and a pressure reducer has been added to deliver the ink to the print head at optimum back pressure.
One complication in the evolution of off-axis printing is the increasing need to maintain the back pressure of the ink at the print head to within as small a range as possible. Changes in back pressure greatly affect print density and print quality, and major changes in back pressure can cause either the ink to drool out of the nozzles or to deprime the print cartridge.
There are several causes for such changes in back pressure. One cause occurs when air is entrapped within the print cartridge and the print cartridge is subjected to changes in environmental parameters such as altitude, acceleration, and temperature. If the air entrapped in a print cartridge acts according to the Ideal Gas Law, PV=nRT, then any changes in any of these parameters will cause corresponding changes in back pressure. Another cause is the delay between the time the print head starts to eject ink during on-demand printing and the time the pressure regulator actuates to restore the back pressure.
These complications as well as the use of pressurized ink delivery have all resulted in a need for more accurate back pressure regulation in ink-jet print cartridges and for more precise compensation techniques.